In an internal-combustion engine of a car, fossil fuels such as gasoline or light-oil are burned. Exhaust gas generated by the burning contains water and carbon dioxide, as well as environmental pollutants such as unburned carbon monoxide (CO), unburned carbon hydride (HC), sulfur oxide (SOx), and nitrogen oxide (NOx). In recent years, various countermeasures to purify the car exhaust gas have been taken especially for environmental protection and prevention of living environment pollution.
As one of such countermeasures, a use of an exhaust gas purification catalyst unit can be exemplified. Specifically, a three-way catalyst for exhaust gas purification is disposed in the middle of an exhaust system, and, there, CO, HC, NOx, etc. are decomposed by oxidation-reduction process to thereby render the above environmental pollutants harmless. In order to maintain the decomposition of NOx in the catalyst unit, urea solution is sprayed to the catalyst from upstream side of the catalyst unit in the exhaust system. In order to enhance the rate of decomposition of NOx, urea concentration of the urea solution should fall within a specified range, and a urea concentration of 32.5% is considered to be optimum.
The urea solution is stored in a urea solution tank installed in a car. In this state, however, concentration may change with time, or unevenness in the concentration distribution may locally occur in the tank. The urea solution which is supplied from the tank to a spray nozzle through a supply pipe by means of a pump is taken from the outlet provided near the bottom portion of the tank in general. Therefore, it is important for the urea solution in such an area to have a predetermined urea concentration, in order to enhance the efficiency of the catalyst unit.
Conventionally, measurement of the concentration of urea in the urea solution has not directly been made. Meanwhile, a technique that uses NOx sensors disposed respectively on the upstream and downstream sides of the catalyst unit in the exhaust system has been made. In this technique, it is determined whether optimum decomposition of NOx has been carried out based on the difference in NOx concentration detected by these sensors. However, this technique aims at measuring the effect of a reduction in the amount of NOx and therefore cannot determine whether or not the liquid is urea solution having a predetermined urea concentration even at the beginning of the spray of urea solution as well as before the spray. Further, the NOx sensor used in such a technique did not have sufficient sensitivity for ensuring spray of urea solution having a urea concentration falling within a predetermined range.
JP-A-11-153561 discloses a fluid identifying method. In this method, a current is applied to heat a heater, and the heat generated is used to heat a temperature sensor. Then, thermal influence is applied to heat transfer from the heater to temperature sensor using a fluid to be identified and, based on an electrical output value of the temperature sensor which corresponds to a resistance value, the type of the fluid to be identified is determined. The application of a current to the heater is periodically performed in this method.
However, although this method can distinguish among substances (e.g., water, air, and oil) having properties largely different from each other using representative values, it has difficulty determining whether or not the liquid to be measured as described above is urea solution having a predetermined urea concentration correctly and quickly.
As a typical application of the thermal sensor, measurement of mass flow rate of a liquid can be exemplified. Description of a thermal flow sensor used in such an application and a flowmeter (flow measurement device) using the thermal flow sensor is disclosed in, e.g., JP-A-11-153465, JP-A-11-153466, JP-A-2002-202166, JP-A-2003-279395, and JP-A-2003-302271.
In the case where the above-described thermal sensor, especially, an indirect-heating thermal sensor as disclosed in the above patent documents is used, if a fluid to be measured is a liquid, air and the like dissolved in the liquid is evaporated by a rise in temperature to form gas bubbles, and the gas bubbles may be adhered to the outer surface of the sensor in some cases. Further, in the case where the liquid to be measured stored in the tank has free surface in the tank, when the liquid in the tank is vibrated, the liquid surface is agitated to cause gas such as air contacting the liquid surface to be caught up in the liquid, with the result that the gas remains in the liquid as gas bubbles, and the gas bubbles may be adhered to the outer surface of the sensor in some cases.
In particular, in the case of urea solution in the tank installed in a car, severe vibration based on an external force is repeatedly applied while the car is moving, so that the adherence of the gas bubbles to the sensor outer surface becomes marked.
The adherence of the gas bubbles to the sensor prevents heat emitted from the heating element from being favorably transferred through a heat transfer member to the liquid, or prevents heat from being favorably transferred from the liquid through the heat transfer member to the temperature sensing element. When the heat transfer between the sensor and liquid to be measured is not performed normally, a large error occurs in the measurement value of the concentration of the liquid to be measured, which may result in remarkable decrease in the reliability of measurement.